This invention relates to elastomers and more particularly to elastomeric compositions comprising a rigid copolymer and a rubbery copolymer and to a method for the preparation thereof. Still more particularly, this invention relates to elastomeric compositions having a core-shell structure prepared by a sequential polymerization process, said compositions comprising (a) a rigid copolymer core formed of at least one rigid monovinylidene monomer and a first copolymerizable functional monomer, said rigid copolymer having a glass transition temperature of greater than 20.degree. C., (b) a rubbery copolymer outer layer or shell formed of at least one rubbery acrylic monomer and a second copolymerizable functional monomer, said rubbery copolymer having a glass transition temperature of less than about 0.degree. C. and (c) a transition layer, intermediate between the core and outer shell, formed of a mixture of the core components and the outer shell components.
Conventional rubbers and synthetic elastomers are generally tacky, low strength gums which require vulcanizing in order to attain useful properties. The vulcanizing or curing process introduces a high degree of crosslinking which in turn insolubilizes these materials and renders them intractable. Cured rubbers are thus no longer thermoplastic and are not readily reprocessable. Thermoplastic hydrocarbon elastomers have recently been introduced which do not require vulcanizing to be useful. Unlike ordinary rubbers, compositions such as the sytrene-butadiene-styrene block copolymers disclosed in Holden, et al, U.S. Pat. No. 3,265,765, possess the necessary elastomeric properties without curing and are permanently thermoplastic and soluble. These and related thermoplastic block copolymers have found wide commercial acceptance in part because of their ready reprocessability. The method of their preparation, however, involves an anionic solution polymerization process which inherently is limited to use with a narrow spectrum of anionically polymerizable hydrocarbon monomers. Because such polymers are readily dissolved by low molecular weight hydrocarbons they do not exhibit good oil and solvent resistance. Additionally, these hydrocarbon elastomers are subject to other environmental attack and particularly to degradation by UV radiation because of the presence of ethylenic unsaturation and are unsuited for outdoor use without substantial stabilizing or further treatment.
Acrylic elastomers have long been known for their heat stability and resistance to oil. These materials are prepared from acrylic esters such as ethyl acrylate and butyl acrylate and must be thermally or chemically crosslinked to impart the necessary strength properties. Emulsion polymerization processes have been widely employed for the production of acrylic elastomers and methods for chemical crosslinking of these materials with difunctional monomers are well known. As with conventional elastomers, these materials are insoluble and difficult to reprocess after being cured. More recently, a thermoplastic acrylic elastomer has become known as was disclosed in Shah, et al, U.S. Pat. No. 4,306,039. These elastomers are blends of a rubbery acrylic copolymer containing an acidic functional monomer with a rigid polymer of an N-vinyl lactam. Although the resulting elastomers are said to be oil resistant, reprocessable thermoplastics, the compositions are limited to blends of acrylic rubbers with N-vinyl lactam polymers. Altering the balance of properties to reduce water sensitivity or change other physical characteristics by the incorporation of additional monomers is not readily achieved.
Elastomers having a core-shell structure have also been disclosed in the prior art. The term core-shell structure has become well-understood in the art as defining a layered particulate composition having a polymeric center or core surrounded and encapsulated by a shell or overcoat formed of a second polymeric material. Methods for the preparation of such core-shell particulate compositions are now well known in the art and include a variety of layered particulate materials having a core and one or more shell layers. For example, in U.S. Pat. No. 3,661,994, graft polymers formed by a sequential polymerization process are disclosed wherein a rigid, polymeric seed or core is surrounded by a graft polymerized rubber layer and, optionally, encapsulated with a graft polymerized rigid outer layer. It will be understood that the core-shell description of these particulate compositions is intended to describe the product of the polymerization process; fusion of the shell component occurs during thermal processing such that the core-shell nature of the product is no longer discernable. The resulting particulate materials are useful as reinforcers and impact modifiers for plastics. Similarly, in U.S. Pat. No. 3,502,745 an acrylic elastomer is prepared by sequentially graft polymerizing monomers to form an acrylic elastomer shell in the presence of and encapsulating a rigid, particulate cross-linked polymeric core. These elastomers may further include complementary reactive monomers in the rigid and rubbery layers that interact by a condensation reaction in a subsequent curing step to bond the layers and effect a further crosslinking of the rubbery layer. The resulting acrylic elastomers are thus cured during processing and are not truly reprocessable.
A thermoplastic composition having elastomeric properties which could be prepared by a low cost emulsion process from readily available monomers and which could be varied over a wide composition range to alter the balance of physical properties to meet varied end use requirements would thus be a substantial advance in the elastomer art.